I'm not at all American.
But....
In simple terms the multipler is like the domino effect of increased government expenditure, and if you work out the multiplier, you can tell how much increasing expenditure by x amount will up the GDP. Its like, if the government spends $100 on a road, then the road making company has an extra $100 of business, so they spend that on something, the person who makes the something will also spend more, etc etc, so the economy benefits by more than just the first $100
The formula to work it out is 1/MPS, just follow that and its all good. e.g If a government has a MPS of 0.2 and wants to know the multiplier effect of spending $100.
The multiplier >1/mps > 1/0.2 >5
Then multiplier x proposed spending > 5 x $100 = $500.
So GDP will increase by $500.
Hope that helps